Stoppers for beverage containers that open and close a liquid through hole that is provided in a stopper main body in a freely rotatable manner by biasing a lid that is axially supported by an end of the stopper main body toward an opening direction using a torsion spring, are widely known. For example, Japanese Unexamined Patent Application, First Publication No. 2001-149236 discloses a stopper for a liquid container in which an internal stopper is provided in a hinged freely rotatable manner using a support shaft that is provided at an end of an external stopper, and the internal stopper is biased toward an opening direction by a torsion spring that is provided on a support shaft. In addition, in the same manner, Japanese Unexamined Patent Application, First Publication No. 2012-106802 discloses a stopper for a beverage container in which a single door cap is axially supported in a rotatable manner by a hinged connection portion that is provided in the stopper, and is biased toward an opening direction by a spring.
However, in both stoppers, the torsion spring or the like that act as biasing means are exposed to the outside, and therefore, in a case in which the torsion spring or the like becomes broken due to metal fatigue or the like, there is a concern that fragments of the torsion spring or the like will fall inside the beverage container. In addition, since the torsion spring or the like is exposed to the outside, it is detrimental to the external appearance.